patch 9.1.0369: Vim9: problem when importing autoloaded scripts

Problem:  Vim9: problem when importing autoloaded scripts
Solution: In `:def` handle storing to vim9 autoload export
          (Ernie Rael)

Problem occurs when `import autoload ./.../autoload/...`. The autoload
in the specified path causes the use of an autoload_prefix which combines
with the `import autoload` to create trouble.

In `generate_store_var()` `case dest_script` use ISN_STOREEXPORT,
when needed, instead of ISN_STORES. When executing ISN_STOREEXPORT,
check for autoload_prefix.

fixes: #14606
closes: #14615

Signed-off-by: Ernie Rael <errael@raelity.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>

src/testdir/test_vim9_func.vim

@@ -166,6 +166,37 @@ def Test_wrong_function_name()
   delfunc g:Define
 enddef
 
+" Check that in a legacy script a :def accesses the correct script variables.
+" Github issue: #14615.
+def Test_access_var_from_legacy_def()
+  # Access a script variable by name WITH "s:" prefix.
+  var lines =<< trim END
+    let s:foo = 'init'
+    let s:xxfoo = 'init'
+    def! AccessVarFromLegacyDef()
+        s:xxfoo = 'CHANGED'
+    enddef
+    call AccessVarFromLegacyDef()
+    call assert_equal('init', s:foo)
+    call assert_equal('CHANGED', s:xxfoo)
+  END
+  v9.CheckScriptSuccess(lines)
+
+  # Access a script variable by name WITHOUT "s:" prefix;
+  # previously this accessed "foo" and not "xxfoo"
+  lines =<< trim END
+    let s:foo = 'init'
+    let s:xxfoo = 'init'
+    def! AccessVarFromLegacyDef()
+        xxfoo = 'CHANGED'
+    enddef
+    call AccessVarFromLegacyDef()
+    call assert_equal('init', s:foo)
+    call assert_equal('CHANGED', s:xxfoo)
+  END
+  v9.CheckScriptSuccess(lines)
+enddef
+
 def Test_listing_function_error()
   var lines =<< trim END
       var filler = 123